The present invention relates to an expandable vinyl chloride resin composition, and more particularly to an expandable vinyl chloride resin composition which is excellent in processability and provides cellular molded articles having a high expansion ratio and having excellent strength and surface property.
Polyvinyl chloride resins provide molded articles excellent in physical properties such as impact resistance and heat resistance and in chemical properties such as solvent resistance, acid resistance and alkali resistance and, therefore, have been widely used in the field of building materials and other various fields. In recent years, foaming method attracts attention as a means for lightening of vinyl chloride resins and cost reduction of molded articles. Cellular molded articles of vinyl chloride resins having a high expansion ratio have been demanded strongly from the market.
In foaming of vinyl chloride resins, generally known is a method using a blowing agent in combination with a processing aid comprising methyl methacrylate as a main component.
It is known that it is possible to achieve foaming in a high expansion ratio when an easily volatile organic solvent blowing agent such as an aliphatic hydrocarbon or a halogenated aliphatic hydrocarbon is used as the blowing agent mentioned above.
For example, Japanese Patent Publications Kokoku No. 60-10540 and Kokoku No.58-40986 disclose that foamed articles having an expansion ratio of several tens of times are obtained by impregnating a vinyl chloride resin with an organic solvent having a boiling point of not more than 90xc2x0 C. such as butane or dichlorofluoromethane as a blowing agent or by directly introducing the organic solvent to an extruder during extrusion processing.
However, the use of an organic solvent blowing agent is disadvantageous in cost as compared with a thermally decomposable blowing agent, since equipments for the impregnation and for explosion proof are required in carrying out the molding.
On the other hand, in case of using a thermally decomposable blowing agent such as a thermally decomposable organic blowing agent or a thermally decomposable inorganic blowing agent, it is difficult under existing circumstances to raise the expansion ratio to more than about 3-4 times so long as it is desired to produce foamed articles having a smooth surface and to keep the cells thereof uniform and fine. If the expansion ratio is raised to more than 4 times, especially to more than 5 times, there arises a problem that the strength and surface property of foamed articles are deteriorated.
For example, Japanese Patent Publication Kokoku No. 63-9540 discloses an expandable vinyl chloride resin composition obtained by adding a methacrylate resin (polymethyl methacrylate having a degree of polymerization of 2,000 to 30,000, that is, a weight average molecular weight of 200,000 to 3,000,000) with a thermally decomposable organic blowing agent such as azodicarbonamide and a thermally decomposable inorganic blowing agent such as sodium bicarbonate and further with a filler such as calcium carbonate to a vinyl chloride resin having an average degree of polymerization of 500 to 800. It is disclosed that this expandable vinyl chloride resin composition is molded to give foamed articles having uniform and fine cells and having excellent surface property and surface hardness, but the expansion ratio is at most about 3-4 times.
Also, Japanese Patent Publication Kokai No. 6-9813 discloses an expandable vinyl chloride resin composition obtained by adding a methacrylate resin and a bicarbonate having a particle size of not more than 10 xcexcm as a thermally decomposable blowing agent to a vinyl chloride resin. It is disclosed that this expandable vinyl chloride resin composition is molded to give foamed articles having uniform and fine cells and having good heat stability and weatherability, but no detail of the average molecular weight of methacrylate resin and the expansion ratio is disclosed therein.
Further, Japanese Patent Publication Kokai No. 9-151269 discloses an expandable vinyl chloride resin composition obtained by adding a polymethyl methacrylate resin having a weight average molecular weight of 4,500,000 to 7,000,000 and a thermally decomposable blowing agent to a vinyl chloride resin. It is disclosed that this expandable vinyl chloride resin composition is molded to give injection-molded foams having uniform cells without occurrence of decomposition of the resins. However, in the working examples thereof, as the polymethyl methacrylate resin is used only a resin xe2x80x9cP-531xe2x80x9d made by Mitsubishi Rayon Co., Ltd. having a weight average molecular weight of 4,700,000, and no evaluation is made with respect to monomer composition and expandability. Also, the obtained expansion ratios are at most about 2-3 times.
Also, Japanese Patent Publication Kokai No. 9-208732 discloses an expandable vinyl chloride resin composition obtained by incorporating a methacrylic ester resin, a chlorinated polyethylene resin, talc and a blowing agent into a vinyl chloride resin. It is disclosed that this expandable vinyl chloride resin composition is molded with good molding processability to give foamed articles having a small linear expansion coefficient and uniform cells. However, the obtained expansion ratios are at most about 2-3 times.
Thus, it is an object of the present invention to provide an expandable vinyl chloride resin composition, according to which the expansion ratio can be markedly increased by the use of a thermally decomposable blowing agent without using any organic solvent-based blowing agent in the foaming, and deterioration of strength and surface property of foamed articles caused by the increase in expansion ratio can be suppressed.
The present inventors have found that a composition capable of greatly increasing the expansion ratio as compared with conventional compositions containing a thermally decomposable blowing agent and capable of improving the strength and surface property of foamed articles is obtained by adding a specific (meth)acrylic acid ester copolymer, a thermally decomposable inorganic blowing agent, a thermally decomposable organic blowing agent and optionally a filler to a vinyl chloride resin.
Thus, in accordance with the present invention, there is provided an expandable vinyl chloride resin composition comprising (1) 100 parts by weight of a vinyl chloride resin, (2) 0.5 to 30 parts by weight of, as a processing aid, a two stage (meth)acrylic acid ester polymer having a specific viscosity of not less than 0.5 measured at 30xc2x0 C. with respect to a solution of 0.1 g of the polymer dissolved in 100 ml of chloroform, the two stage polymer being obtained by emulsion-polymerizing (a) 50 to 99 parts by weight of a monomer mixture comprising 50 to 100% by weight of methyl methacrylate, 0 to 50% by weight of at least one monomer selected from the group consisting of an acrylic acid ester and a methacrylic acid ester excepting methyl methacrylate and 0 to 20% by weight of other vinyl monomer copolymerizable therewith to give a polymer having a specific viscosity of not less than 0.7 measured at 30xc2x0 C. with respect to a solution of 0.1 g of the polymer dissolved in 100 ml of chloroform and polymerizing, in the presence of a latex of the obtained polymer, (b) 1 to 50 parts by weight of a monomer mixture comprising 0 to 50% by weight of methyl methacrylate, 50 to 100% by weight of at least one monomer selected from the group consisting of an acrylic acid ester and a methacrylic acid ester excepting methyl methacrylate and 0 to 20% by weight of other vinyl monomer copolymerizable therewith wherein the total of the monomer mixtures (a) and (b) is 100 parts by weight, (3) 0.3 to 25 parts by weight of a thermally decomposable inorganic blowing agent and (4) 0.01 to 15 parts by weight of a thermally decomposable organic blowing agent.
Sodium bicarbonate is particularly preferred as the thermally decomposable inorganic blowing agent.
The expandable vinyl chloride resin composition of the present invention may further contain 1 to 20 parts by weight of a filler. As a filler are preferably used calcium carbonate, talc, mica, clay, wollastonite, asbestos, glass fiber and montmorillonite.
The feature of the present invention resides in that a copolymer which is obtained by emulsion polymerization of a monomer mixture containing a predominant amount of specific methacrylic acid ester and/or acrylic acid ester is used as a processing aid for vinyl chloride resins. By using such a processing aid together with both a thermally decomposable inorganic blowing agent and a thermally decomposable organic blowing agent or with them and a filler, there can be exhibited the effect that the expansion ratio in the foaming can be increased without impairing excellent physical and chemical properties that the vinyl chloride resins originally possess.
The vinyl chloride resins used in the present invention are not particularly limited, and any of conventionally used vinyl chloride resins can be used in the present invention. Homopolymer and copolymers composed of 80 to 100% by weight of units of vinyl chloride and 0 to 20% by weight of units of other monomers copolymerizable with vinyl chloride are preferable.
Examples of the other monomer copolymerizable with vinyl chloride are, for instance, vinyl acetate, propylene, styrene, an acrylic acid ester (e.g., alkyl acrylates having a C1 to C8 alkyl group such as methyl acrylate, ethyl acrylate, butyl acrylate and octyl acrylate), and other vinyl monomers. These vinyl monomers may be used alone or in admixture thereof.
The average degree of polymerization of the vinyl chloride resin is not particularly limited, but vinyl chloride resins having an average degree of polymerization of about 400 to about 1,000 are usually employed.
Such vinyl chloride resins include, for instance, polyvinyl chloride, copolymers of not less than 80% by weight of units of vinyl chloride and not more than 20% by weight of units of other copolymerizable monomer such as vinyl acetate, propylene, styrene or an acrylic acid ester, a chlorinated polyvinyl chloride, and the like. These may be used alone or in admixture thereof.
The processing aid used in the present invention comprises a polymer mixture obtained by polymerizing a monomer mixture (b) in a latex of a first stage polymer obtained by emulsion polymerization of a monomer mixture (a). Usually, a composite polymer having a two layer structure is formed by such a polymerization method. The processing aid is used for the purpose of improving the expandability of the vinyl chloride resin.
The monomer mixture (a) is required to contain 50 to 100% by weight, preferably 60 to 90% by weight, more preferably 70 to 85% by weight, of methyl methacrylate. The monomer mixture (a) may contain 0 to 50% by weight of at least one monomer selected from an acrylic acid ester and a methacrylic acid ester excepting methyl methacrylate, and preferably the mixture contains such a monomer in an amount of 10 to 40% by weight, especially 15 to 30% by weight. If the proportion of methyl methacryalte in the monomer mixture (a) is less than 50% by weight, the transparency and expandability are lowered. Also, if the proportion of the monomer selected from an acrylic acid ester and a methacrylic acid ester excepting methyl methacrylate exceeds 50% by weight, the transparency and expandability are lowered. The monomer mixture (a) may further contain 0 to 20% by weight, preferably at most 10% by weight, more preferably at most 5% by weight, of other vinyl monomers copolymerizable with the (meth)acrylic acid esters.
Examples of the methacrylic acid ester excepting methyl methacrylate in the monomer mixture (a) are, for instance, alkyl methacrylates having a C2 to C8 alkyl group such as ethyl methacrylate, propyl methacrylate, butyl methacrylate and 2-ethylhexyl methacrylate. Examples of the acrylic acid ester are, for instance, alkyl acrylates having a C1 to C8 alkyl group such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate and 2-ethylhexyl acrylate. These acrylates and methacrylates excepting methyl methacrylate may be used alone or in admixture thereof.
Examples of the other vinyl monomers in the monomer mixture (a) are, for instance, an aromatic vinyl compound such as styrene or xcex1-methylstyrene, an unsaturated nitrile compound such as acrylonitrile, and the like. These may be used alone or in admixture thereof.
The homopolymer and copolymers (first stage polymer) obtained by emulsion polymerization of the monomer mixture (a) is preferred to have a high molecular weight, and the specific viscosity thereof measured at 30xc2x0 C. with respect to a solution of 0.1 g of the polymer dissolved in 100 ml of chloroform is at least 0.7, preferably from 0.7 to 1.9, more preferably from 0.8 to 1.8, further preferably from 0.8 to 1.7, still further preferably from 0.9 to 1.6. If the specific viscosity is less than 0.7, the expansion ratio cannot be increased. Also, if the specific viscosity exceeds 1.9, the expandability and processability tend to lower.
In the present invention, a monomer mixture (b) is polymerized in the presence of a first stage polymer latex to provide an outer layer of a homopolymer or copolymer made of the monomer mixture (b) on the first stage polymer. By the provision of outer layer of such a second stage polymer on the first stage polymer, gelation of vinyl chloride resins can be accelerated and generation of non-gelled material can be prevented when the obtained two stage polymer is incorporated as a processing aid into the vinyl chloride resins. As a result, viscosity and elasticity can be efficiently imparted to vinyl chloride resins.
The monomer mixture (b) is required to contain 50 to 100% by weight, preferably 51 to 80% by weight, more preferably 55 to 70% by weight, of at least one monomer selected from an acrylic acid ester and a methacrylic acid ester excepting methyl methacrylate. The monomer mixture (b) may contain 0 to 50% by weight of methyl methacrylate, and preferably the monomer mixture (b) contains 20 to 49% by weight, especially 30 to 45% by weight, of methyl methacrylate. If the proportion of the monomer selected from an acrylic acid ester and a methacrylic acid ester excepting methyl methacrylate is less than 50% by weight, a good gelling property is lost and non-gelled material is easy to be produced. In addition, the expandability is also lowered.
Also, the monomer mixture (b) may contain 0 to 20% by weight, preferably at most 10% by weight, more preferably at most 5% by weight, of other vinyl monomers copolymerizable with the above-mentioned (meth)acrylic acid esters. If the content of the copolymerizable other vinyl monomers exceeds 20% by weight, the transparency and expandability are lowered.
Examples of the methacrylic acid ester excepting methyl methacrylate in the monomer mixture (b) are, for instance, alkyl methacrylates having a C2 to C8 alkyl group such as ethyl methacrylate, propyl methacrylate, butyl methacrylate and 2-ethylhexyl methacrylate. Examples of the acrylic acid ester are, for instance, alkyl acrylates having a C1 to C8 alkyl group such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate and 2-ethylhexyl acrylate. These acrylates and methacrylates excepting methyl methacrylate may be used alone or in admixture thereof. Of these, butyl acrylate is preferable from the viewpoint that polymers having a low glass transition temperature are obtained.
Examples of the copolymerizable other vinyl monomers in the monomer mixture (b) are, for instance, an aromatic vinyl compound such as styrene or xcex1-methylstyrene, an unsaturated nitrile compound such as acrylonitrile, and the like. These may be used alone or in admixture thereof.
With respect to the proportions of the monomer mixture (a) and the monomer mixture (b) used in the preparation of the processing aid of the present invention, per 100 parts by weight of the total of the monomer mixtures (a) and (b), the monomer mixture (a) is used in an amount of 50 to 99 parts by weight, preferably 60 to 95 parts by weight, more preferably 65 to 90 parts by weight, and the monomer mixture (b) is used in an amount of 1 to 50 parts by weight, preferably 5 to 40 parts by weight, more preferably 10 to 35 parts by weight.
If the amount of the monomer mixture (a), namely the first stage polymer, is less than 50 parts by weight, the gelation of vinyl chloride resins is not sufficiently improved, so sufficient expandability is not obtained. Also, if the amount of the monomer mixture (b), namely the second stage polymer, is more than 50 parts by weight, gelation property, transparency and expandability of vinyl chloride resin compositions are impaired. By the presence of a polymer formed from the monomer mixture (b) as an outer layer on the first stage polymer, the gelation property, processability and expandability are peculiarly improved and, as a result, the effect of addition of the processing aid of the present invention becomes highly efficient.
The processing aid of the present invention is prepared by the following method.
Firstly, the monomer mixture (a) is emulsion-polymerized in a suitable medium in the presence of an emulsifier, a polymerization initiator and optionally a chain transfer agent according to a known method, thereby giving a first stage polymer latex. The polymerization is conducted by emulsion polymerization method, since it is possible to make the specific viscosity high. Subsequently, the monomer mixture (b) is added to the first stage polymer latex and is polymerized. By such a stepwise polymerization of the monomer mixtures (a) and (b) is produced a two stage polymer comprising an inner layer of a polymer formed from the monomer mixture (a) and an outer layer of a polymer formed from the monomer mixture (b).
The dispersion medium used in the emulsion polymerization is usually water.
Known emulsifiers are used. Examples of the emulsifier are, for instance, an anionic surfactant such as a fatty acid salt, an alkyl sulfate, an alkylbenzene sulfonate, an alkyl phosphate or a sulfosuccinic acid diester, and a non-ionic surfactant such as a polyoxyethylene alkyl ether or a polyoxyethylene fatty acid ester.
As the polymerization initiator are used water-soluble and oil-soluble polymerization initiators. For example, usual inorganic polymerization initiators such as a persulfate, organic peroxides or azo compounds may be used alone, or may be used as a redox system in combination with a sulfite, a thiosulfate, a primary metal salt, formaldehyde sodium sulfoxylate or the like. Preferable persulfates include, for instance, sodium persulfate, potassium persulfate, ammonium persulfate, and the like. Preferable peroxides include, for instance, t-butyl hydroperoxide, cumene hydroperoxide, benzoyl peroxide, lauroyl peroxide, and the like.
The chain transfer agent is not particularly limited. For instance, t-dodecylmercaptan, n-dodecylmercaptan, t-decylmercaptan, n-decylmercaptan and the like can be used.
The temperature and time of the polymerization reaction are not particularly limited and are suitably adjusted so as to obtain the desired specific viscosity and particle size in accordance with the purpose of use.
In the second stage polymerization, the monomer mixture (b) can be added after confirming that the first stage polymerization is completed, whereby the second stage polymerization can be carried out without mixing of the second stage monomer mixture (b) with the first stage monomer mixture (a).
Preferably the polymerization of the second stage monomer mixture (b) is carried out by an emulsion polymerization method from the viewpoint that it is possible to raise the specific viscosity. The emulsifier, polymerization initiator and chain transfer agent to be used in the second stage polymerization are not particularly limited. Also, the polymerization reaction conditions such as time and temperature can be suitably adjusted according to the purpose of use.
The thus obtained two stage polymer latex has an average particle size of 100 to 3,000 xc3x85 (0.01 to 0.3 xcexcm), preferably 100 to 2,000 xc3x85 (0.01 to 0.2 xcexcm). The particles are taken out of the latex by salting out or coagulation by means of addition of a usual electrolyte, or spray drying in hot air. Also, as occasion demands, washing, dehydration, drying and the like are carried out in a usual manner.
The thus obtained two stage polymer useful as a processing aid has a specific viscosity of at least 0.5 measured at 30xc2x0 C. with respect to a solution of 0.1 g of the polymer dissolved in 100 ml of chloroform, preferably 0.5 to 1.7, more preferably 0.6 to 1.6, the most preferably 0.7 to 1.5. If the specific viscosity is less than 0.5, the expansion ratio cannot be increased. If the specific viscosity is more than 1.7, the expandability and processability tend to lower.
In general, the processing aid is preferably a white powder having an average particle size of 30 to 300 xcexcm from the viewpoint of incorporating it as a processing aid into a vinyl chloride resin.
The processing aid is used in an amount of 0.5 to 30 parts by weight, preferably 5 to 25 parts by weight, more preferably 8 to 25 parts by weight, per 100 parts by weight of a vinyl chloride resin. If the amount of the processing aid is less than 0.5 part by weight, the effect to be produced by the addition of the processing aid is not sufficiently obtained. If the amount is more than 30 parts by weight, excellent mechanical properties of the vinyl chloride resin are impaired.
In the present invention, combinations of a thermally decomposable inorganic blowing agent and a thermally decomposable organic blowing agent is used as a blowing agent. The term xe2x80x9cthermally decomposable inorganic blowing agentxe2x80x9d as used herein means an inorganic compound capable of generating, by the thermal decomposition thereof, a gas such as nitrogen gas, carbon dioxide gas, carbon monoxide gas, ammonia gas, oxygen gas, hydrogen gas or mixtures thereof in an amount effective for expansion of vinyl chloride resins. Also, the thermally decomposable organic blowing agent is non-organic-solvent type blowing agents and means an organic compound capable of generating, by the thermal decomposition thereof, an effective amount of a gas, typically a gas composed mainly of nitrogen gas, for the expansion of vinyl chloride resins.
Examples of the thermally decomposable inorganic blowing agent used in the present invention are, for instance, sodium bicarbonate, potassium bicarbonate, ammonium bicarbonate, sodium carbonate, ammonium carbonate, and the like. These may be used alone or in admixture thereof. Of these, sodium bicarbonate is preferred from the viewpoints of expansion efficiency and cost.
The amount of the thermally decomposable inorganic blowing agent is selected in accordance with the purpose without particular restriction. Usually, the amount is from 0.3 to 25 parts by weight, especially 1 to 18 parts by weight, per 100 parts by weight of a vinyl chloride resin. If the amount of the thermally decomposable inorganic blowing agent is less than 0.3 part by weight, foamed articles having a sufficient expansion ratio are hard to be obtained. If the amount is more than 25 parts by weight, foamed articles having uniform cells are hard to be obtained.
Also, the amount of the thermally decomposable inorganic blowing agent may be changed in conformity with the amount of the processing aid. For example, it is preferable to use the thermally decomposable inorganic blowing agent in an amount of, per 100 parts by weight of a vinyl chloride resin, 0.5 to 5 parts by weight when the processing aid is used in an amount of 5 parts by weight, 1.0 to 8 parts by weight when the processing aid is used in an amount of 10 parts by weight, and 1.5 to 18 parts by weight when the processing aid is used in an amount of 20 parts by weight.
Examples of the thermally decomposable organic blowing agent used in the present invention are, for instance, a nitroso compound such as N,Nxe2x80x2-dinitrosopentamethylenetetramine or N,Nxe2x80x2-dimethylN,Nxe2x80x2-dinitrosoterephthalamide, an azo compound such as azodicarbonamide or azobisisobutyronitrile, a sulfonyl hydrazide compound such as benzenesulfonyl hydrazide or toluenesulfonyl hydrazide, and the like. These may be used alone or in admixture thereof. Of these, azodicarbonamide is preferred from the viewpoints of expansion efficiency and cost.
The amount of the thermally decomposable organic blowing agent is from 0.01 to 15 parts by weight, preferably 0.1 to 5 parts by weight, per 100 parts by weight of a vinyl chloride resin. If the amount of the thermally decomposable organic blowing agent is less than 0.01 part by weight, the effect of improving the surface property and strength of foamed articles is not obtained. If the amount is more than 15 parts by weight, the expansion ratio tends to rather lower.
Also, the amount of the thermally decomposable organic blowing agent may be changed in conformity with the amount of the thermally decomposable inorganic blowing agent. For example, it is preferable to use the thermally decomposable organic blowing agent in an amount of, per 100 parts by weight of a vinyl chloride resin, 0.1 to 0.6 part by weight when the inorganic blowing agent is used in an amount of 2 parts by weight, 0.5 to 1.5 parts by weight when the inorganic blowing agent is used in an amount of 5 parts by weight, and 1.0 to 3 parts by weight when the inorganic blowing agent is used in an amount of 10 parts by weight. In general, the inorganic blowing agent/organic blowing agent ratio is from 1:0.05 to 1:0.3 by weight, especially 1:0.1 to 1:0.3 by weight.
It is desirable to further incorporate a filler into the expandable vinyl chloride resin composition of the present invention in order to improve the strength of molded articles.
Examples of the filler used in the present invention are, for instance, calcium carbonate, talc, mica, clay, wollastonite, asbestos, glass fiber, montmorillonite, and the like. These may be used alone or in admixture thereof. Calcium carbonate, talc, mica, wollastonite, glass fiber and montmorillonite are preferred from the viewpoint of improvement in strength.
The amount of the filler is from 1 to 20 parts by weight, preferably 3 to 18 parts by weight, more preferably 5 to 15 parts by weight, per 100 parts by weight of a vinyl chloride resin. If the amount of the filler is less than 1 part by weight, the effect of making the cells of foamed articles uniform is small. If the amount is more than 20 parts by weight, the melt viscosity tends to rise and also the expandability tends to lower.
The expandable vinyl chloride resin composition of the present invention may contain one or more of other additives such as stabilizer, lubricant, impact modifier, plasticizer, colorant and the like, as occasion demands.
The process for preparing the expandable vinyl chloride resin composition of the present invention is not particularly limited. For example, the composition can be prepared in such a manner as mixing a vinyl chloride resin, the processing aid, a thermally decomposable inorganic blowing agent, a thermally decomposable organic blowing agent, and optionally a filler and other additives, and melt-kneading the mixture at an appropriate temperature by a melt kneader such as a single screw extruder or a twin screw extruder.
The method of the molding processing of the expandable vinyl chloride resin composition of the present invention is not particularly limited, and generally used molding methods such as extrusion are applicable.
The present invention is more specifically explained by means of examples and comparative examples, in which all parts and % are by weight unless otherwise noted. It is to be understood that the present invention is not limited to these examples.
Methods of evaluation used in the examples and comparative examples are shown below.
(Measurement of Specific Viscosity of Polymer)
In 100 ml of chloroform was dissolved 0.1 g of a polymer sample, and the measurement was carried out using a Ubbelohde""s viscometer kept at a constant temperature in a water bath of 30xc2x0 C.
(Measurement of Expansion Ratio)
After measuring the specific gravity of a obtained powder compound (non-foamed molding of vinyl chloride resin composition), the compound was molded by a molding machine Model CMT-45 made by Cincinnati Milacron Inc. to give a plate-like molded article (foamed molding of vinyl chloride resin composition), and the specific gravity of the plate-like molded article was measured. The expansion ratio was calculated from the measured values according to the following equation.
Expansion ratio=(specific gravity of non-foamed molding)/(specific gravity of foamed molding) 
The molding conditions are shown below.
Molding conditions
Molding temperature:
C1=150xc2x0 C., C2=160xc2x0 C., C3=170xc2x0 C.
Adapter=175xc2x0 C., Die=180xc2x0 C.
Revolutions of screw: 10 r.p.m.
Output: 15 kg/hour
Die: 1 mmxc3x97170 mm
(Measurement of Polymerization Conversion)
The polymerization conversion was calculated according to the following equation.
Polymerization conversion (%)=(amount of produced polymer/amount of monomers charged)xc3x97100 
(Measurement of Average Particle Size of Latex)
With respect to a latex obtained, the average particle size was measured using light scattering in a wavelength of 546 nm by a Spectrophotometer U-2000 made by Hitachi, Ltd.
(Evaluation of Surface Property)
With respect to the surface property of a foamed article obtained, the appearance was visually observed and evaluated according to the following criteria.
A: Unevenness of the surface is scarcely observed and the appearance is excellent.
B: Unevenness of the surface is observed but is not so noticeable.
C: The surface is uneven and the appearance is somewhat bad.
D: Unevenness of the surface is marked and the appearance is bad.
(Measurement of Compressive Strength)
Compressive strength at 20% compression was measured at 23xc2x0 C. according to JIS K 7220.
(Measurement of Flexural Strength)
Flexural strength was measured at 23xc2x0 C. according to JIS K 7221.
(Measurement of Heat Distortion Temperature)
Heat distortion temperature (HDT) was measured under a condition of bending stress 18.5 kgf/cm2 according to JIS K 7207.